The present invention generally concerns a differential oscillator circuit including an electromechanical resonator, in particular for an application as a timekeeper or in the field of telecommunications.
Oscillator circuits are used in two main application categories. One of these applications is as a timekeeper or clock signal generator and the other is as a circuit for making signal frequency translation possible in telecommunication devices.
Electro-mechanical resonators, such as quartz resonators, are used, so to speak, systematically in applications as time-keeper, requiring a precise time base, whereas the oscillator circuits used in radio-transmitters generally use LC tank circuits. There is no substantial difference in the circuits associated with these two types of resonators.
FIG. 1a shows a low consumption oscillator circuit 1 including a quartz resonator typically used in applications as time-keeper. This oscillator circuit 1, also known as a Colpitts oscillator circuit, thus includes a branch including a series arrangement, starting from a supply potential VDD to a supply potential VSS forming earth, from a current source 2 and a MOS transistor 4 connected via its drain terminal to current source 2 and via its source terminal to potential VSS. A quartz resonator 6 and a resistive element R are connected in parallel between the connection node, indicated by the reference A, of current source 2 and transistor 4, and the connection node, indicated by the reference B, connected to the gate terminal of transistor 4. First and second charge capacitive elements C1 and C2 are respectively connected, via one of their electrodes, to connection nodes A and B, the other electrode being connected to supply potential VSS.
FIG. 1b shows a differential version of a similar oscillator circuit to the circuit of FIG. 1a. In this example, the circuit uses an LC tank circuit. This oscillator circuit, also globally indicated by the reference numeral 1, includes, placed in parallel between supply potentials VDD and VSS, first and second branches 10, 20 each including a series arrangement of a current source 2, respectively 3, and a transistor 4, respectively 5, connected by its drain terminal to the current source and via its source terminal to potential VSS. The LC tank circuit includes a capacitive element C placed in parallel to the series arrangement of a resistive element R, generally symbolising the arrangement losses, and an inductive element L.
Transistors 4 and 5 are connected in a differential configuration so as to form a crossed pair, i.e. the gate terminal of each transistor is connected to the drain terminal of the other transistor. Connection node A is thus formed of the connection node between current source 2, the drain terminal of transistor 4 and the gate terminal of transistor 5, and connection node B is formed of the connection node between current source 3, the drain terminal of transistor 5 and the gate terminal of transistor 4. The LC tank circuit is thus placed, in a similar manner to the quartz resonator of FIG. 1a, between connection nodes A and B of the oscillator circuit, on the side of the drain terminals of transistors 4 and 5.
The differential structure of FIG. 1b offers substantial advantages for high frequency applications, such as, particularly, reduced sensitivity to the supply and substrate noise, reduced harmonic pair content and limited substrate current injection.
It should be mentioned that the circuit of FIG. 1b cannot be used as such with an electromechanical resonator, such as a quartz resonator, since this circuit would exhibit, in such case, instability of the continuous DC component, except if a path with low ohmic value existed at low frequency between the drain terminals of the transistors such as in the parallel LC tank circuit configuration.
Differential oscillator circuit embodiment attempts using an electro-mechanical resonator have been proposed but have not, as yet, led to satisfactory solutions, mainly for reasons of stability. FIG. 1c shows, for example, the prototype of a differential oscillator circuit employing a quartz resonator developed for the first electronic Swiss watch.
This circuit prototype includes two identical branches 10, 20 each including, starting from the supply potential VDD to the supply potential VSS, the series arrangement of a resistive element 8, respectively 9, of an n-MOS transistor 4, respectively 5, and a current source 2, respectively 3. The quartz resonator 6 is connected on the side of the source terminals of transistors 4 and 5 and, in a similar way to the circuit of FIG. 1b, transistors 4 and 5 are connected in a differential configuration, the gate terminal of each transistor being connected to the drain terminal of the other transistor.
As already mentioned, making the differential oscillator circuit of FIG. 1c has not been brought to a successful conclusion for reasons of stability.
In addition to the aforementioned oscillator circuits, it will be noted that recent developments in the manufacture of bulk acoustic wave resonators (or BAW resonators), offer new opportunities for applications in the field of telecommunications. Electro-mechanical BAW resonators, and more particularly, thin film BAW resonators have considerable advantages, in particular high working frequencies (of the order of 1 to 10 GHz), a high quality factor, reduced size and the possibility of being integrated directly onto the integrated circuit. By way of complementary information concerning BAW resonators, reference could be made to the document by MM. K. M. Lakin, K. T. McCarron and R. E. Rose, xe2x80x9cSolidly Mounted Resonators and Filtersxe2x80x9d, 1995 IEEE Ultrasonics Symposium, pp. 905-908.
The possibilities offered by the aforementioned BAW resonators require the development of dedicated circuits making use of all the advantages and excellent properties of these resonators.
One general object of the present invention is to propose an oscillator circuit which is in particular suited, but not solely, to high frequency applications.
Another object of the present invention is to propose such an oscillator circuit, which also has good stability.
Another more particular object of the present invention is to propose an oscillator circuit able, in particular, to make use of the advantages and excellent properties of the aforementioned BAW resonators.
The present invention thus concerns a differential oscillator circuit including an electromechanical resonator whose features are listed in the independent claim 1.
Advantageous embodiments of the present invention form the subject of the dependent claims.
According to the invention, the differential oscillator circuit includes, in particular, first and second branches each including a series arrangement of at least a first current source, a transistor connected via its current terminals and resistor means assuring adequate polarisation of the transistors. The transistors are connected, in a similar way to the prior art, in a differential configuration, the control terminal of each transistor being connected to the current terminal of the other transistor having the most positive potential, like the drain terminal of the transistors in the event that the latter are made in MOS technology.
More particularly, according to the invention, the electromechanical resonator is connected between the control terminals of the transistors, namely on the side of the current terminals of the transistors having the most positive potential, like the drain terminals of MOS technology transistors. Moreover, a capacitive element is placed between the current terminals of the transistors having the most negative potential, namely the source terminals of MOS transistors.
The various embodiments, which will be described in the following description, are based on MOS technology. Although not explicitly shown, the dual versions of these embodiments (obtained by reversing the supply voltages and the polarities of the transistors) are also deemed to form part of the invention. Furthermore, it will likewise be understood that the present invention is not limited to this specific technology and that the transistors could, if required, by made in bipolar technology.
The invention proposes, for the first time, an efficient solution relying on an oscillator circuit with a differential structure including an electromechanical resonator. Unlike the differential resonator of FIG. 1c which has a stability problem, the oscillator circuit according to the present invention has great stability and essentially behaves like the oscillator circuit of FIG. 1b in proximity to the resonant frequency of the LC tank circuit. The capacitive element placed on the source side with respect to the crossed pair of transistors fixes the frequency below which the stability of the continuous DC componentxe2x80x94or negative counter-reactionxe2x80x94dominates, and above which oscillation occurs.
According to the invention, the current sources can regulate the oscillation amplitude if they are placed in a low frequency regulating loop following an amplitude detector.
From the point of view of consumption, the oscillator circuit according to the present invention performs better than the Colpitts oscillator illustrated in FIG. 1a, since the charge capacitive elements C1 and C2 placed on either side of the Colpitts resonator are omitted, according to the present invention, without modifying the behaviour of the circuit and without being detrimental to frequency stability of the oscillator.